That would be even better at 20%.

I was looking around on good ol craigslist and found some steals on steel.
http://stlouis.craigslist.org/mat/1026884935.html
So I was thinking 6" channel with 3" angle crossmembers. A stick of 5" at the yard was $112 for a 20fter. 6" from this guy was $60 before haggling. By the way if you ever need a quick steel quote use this website
http://www.metalsdepot.com/index.phtml?aident=
Shipping is crazy but I have ordered small quantities from them before.